Bistable nematic liquid crystal display device with improved optical mask, having textures differing by about 180° twist, and off state in the liquid crystal covering the inter-electrode spaces

ABSTRACT

The invention relates to a bistable nematic liquid-crystal device comprising: alignment means, which orient the liquid crystal, means designed to apply electrical signals which allow switching, by breaking the anchoring on at least one of the two substrates, between two distinct textures, and means designed to promote the establishment of that one of the said textures giving the off state in the liquid crystal covering the inter-electrode spaces.

The present invention relates to the field of liquid-crystal displays.

More specifically, the present invention relates to bistableanchoring-breaking nematic liquid-crystal displays, two stable texturesof which differ by a 180° twist.

Liquid-crystal displays are widely used for the displaying of symbols,text and images by computers, video projectors, television sets, watchesand many other systems for presenting information. Their surface isusually divided into small elements (pixels) which form cells, thestates of which are independent.

A very important quality criterion of a liquid-crystal display isoptical contrast, this generally being defined by a quantity called thecontrast or contrast ratio. The contrast of a liquid-crystal display isthe ratio of the intensity of the light transmitted or reflected by aregion of the screen placed in the on state to the intensity of thelight transmitted or reflected by the same region placed in the offstate (whether the intensity of the transmitted light or the reflectedlight is considered depends on whether the display is designed foroperation in transmission or in reflection). The on state of the displayis ideally characterized by as high as possible a degree of transmissionor reflection, whereas the off state ideally has as low as possible adegree of transmission or reflection. A high contrast gives theinformation or images displayed a greater readability and betterquality. For example, a contrast ratio of less than 10 corresponds to alacklustre or washed-out image. The same image presented with a contrastof 20 appears much brighter and sharper. The region of the display takeninto account for measuring the contrast may incorporate many pixels ifthe size of the latter is less than the eye's limit of resolution underthe normal conditions of observing the display.

A liquid-crystal display generally comprises, as illustratedschematically in the appended FIGS. 1 to 3, two parallel plates 12, 22provided with respective electrodes 10, 20 on their inner surfaces andcontaining a liquid-crystal material 30 between the aforementionedelectrodes. The regions of crossover of the opposed electrodes 10, 20define the pixels 40. The displayed image is formed from the mass ofthese pixels placed in the desired optical states. The optical state ofeach pixel, i.e. whether it is on or off, is determined by theapplication of suitable electric fields between the electrodes 10, 20which define the pixels and by the addressing circuit used.

Mostly liquid-crystal displays are monostable. In these displays, asingle texture is produced with no field. In a field, this textureprogressively varies, giving increasingly bright (or increasingly off)states.

Bistable displays possess at least two states which remain stable for atheoretically infinite time in the absence of an applied field. One ofthe states is the off state and the other the on state. The stability ofthe two states stems from the fact that the energies of the two texturesare practically equal and that there exists a potential barrierpreventing one texture from spontaneously switching to the other.

Adjacent pixels are not contiguous but separated by a non-addressablearea called the inter-pixel area (see FIG. 3). Shown schematically inFIG. 3 are a pixel with the reference P and an inter-pixel gap with thereference I. When, for example, the display is organized in the form ofa passive matrix, the row electrodes 10 are attached to a substrate 12,the column electrodes 20 are attached to another substrate 22 and theelectrodes 10 and 20 intersect at right angles. In this case, the pixelsP are bounded by the areas in which the row electrodes 10 and columnelectrodes 20 overlap and the inter-pixel gaps I correspond to the areasfacing an inter-electrode space, whether this be on the row substrate 12or the column substrate 22.

This inter-electrode space allows each electrode to be isolated from itsneighbours and it cannot be reduced to below a critical value whichdepends on the nature and the thickness of the material used to form theelectrodes, on the etching technology employed, on the electric fieldsto be applied and on other characteristics of the display.

In the absence of a blocking layer, the optical state of the inter-pixelgap is poorly defined and the transmission or reflection of the light bythese regions is likely to considerably degrade the contrast of adisplay. For example, a display organized in square pixels of 0.25 mmpitch and having sides of 0.23 mm would have an inter-pixel gap of 0.02mm in width, corresponding to a total non-active area of about 15% ofthe total working area of the screen. In the unfavourable situation ofan inter-pixel gap always being on, the contrast of the display measuredin a region encompassing several pixels could drop to a value of lessthan 7, even if the microscopic contrast measured within the pixels isarbitrarily high.

The use of a negative-contrast display mode (i.e. a mode having an offstate when the field is zero, for example the TN mode in parallelpolarizers) makes it possible to limit the loss of contrast comparedwith a positive-contrast display. However, this solution is notapplicable to high-resolution displays for which the lateral fieldscreated when addressing the pixels distort the texture of the liquidcrystal between adjacent electrodes and turn it to the optically onstate [1]. Choosing a negative contrast may also require the use of anunfavourable optical configuration (the off state of a TN with negativecontrast is for example inferior to that of a TN with positive contrast,because of its chromatism), thereby limiting the contrast of thedisplay.

All the other solutions proposed for mitigating the loss of contrast dueto the non-active portions of the screen are based on the additionbetween the pixels or in the inter-electrode space of an opaque layerwhich blocks the transmitted or reflected light.

Document [2] proposes the use as optical mask of aluminium or chromiumlayers. The use of metals as optical mask requires an expensiveadditional vacuum deposition operation and isolation precautions toprevent the appearance of short circuits between pixels. These layersalso have an irreducible degree of reflection of about 1.5 to 3%,thereby limiting the contrast in reflection.

Document [3] proposes the use of an absorbent (coloured or black)polymer as optical mask. However, the rather low optical density ofcoloured polymers requires their use as a relatively thick layer (>0.5μm) in order to obtain a sufficiently low degree of transmission (<1%).Etching these layers therefore results in the creation of an abruptinternal relief within the display and the concomitant appearance ofnon-uniform electric fields, which induce orientational defects such astilt-inversion or twist-inversion regions. These defects may be reducedif the optical mask is enlarged to partially cover the edges of theelectrodes, to the detriment of the degree of transmission or reflectionin the on state. Moreover, the precision in etching these quite thickpolymer layers is poor, which means that the optical mask must bewidened in order to maintain sufficient alignment tolerances between theoptical mask and the electrodes. This method therefore maintains thecontrast, but to the detriment of another important characteristic ofthe display, which is the brightness of the luminous state.

Another solution is to use a blocking layer made of a stack of a metaland of an absorbent polymer layer providing an antireflection andisolation function [4] so as to optimise the optical properties of theoptical mask in transmission and in reflection, without excessivelythickening it, or else to make use of even more complicated structuresbased on dielectrics containing very fine metal or semiconductorparticles having absorbent properties [5].

All these methods involve additional manufacturing operations and theuse of costly materials or processes. This is because it is necessary todeposit the opaque material on the electrodes and remove it from theactive regions. These operations must be precise since it is necessaryto create opaque bands 0.01 or 0.02 mm in width without encroaching onthe pixels or leaving non-blocked regions. The additional cost incurredis particularly prohibitive in the case of passive matrix displays, thereference price of which is that of STN-LCD monochromatic displays andthe manufacturing process of which does not use vacuum deposition.

The present invention makes it possible, in the case of bistable nematicliquid-crystal cells, to block off the inter-pixel spaces without addingmaterial, by modifying or adapting the manufacturing operations. It usesthe properties of bistable displays, which properties will be brieflyreviewed.

For example, document [6] proposes a bistable display whose states areproduced with the same flat or almost flat positions of the molecules onthe substrates (monostable anchoring). These states differ by thetexture of the liquid crystal between the two surfaces of thesubstrates. In one of the states, the molecules are practically parallel(the texture is uniform or slightly twisted). In the other state, themolecules rotate flat by a half-turn, going from one substrate to theother (the texture is twisted through 180° with respect to the previoustexture). Switching between the two textures is obtained in a strongfield by breaking the monostable anchoring on at least one of thesurfaces and by using a hydrodynamic effect.

In the present invention, the inter-pixel spaces are blocked off byproducing bistable cells such that the liquid crystal in the inter-pixelspaces is permanently in the off state. This is accomplished by settingthe anchoring and spontaneous twist conditions specific to the layer ofliquid crystal covering the inter-pixel spaces in order therein tofavour the texture giving the off state, these conditions reducing itsenergy compared with that of the texture giving the on state.

In the text below, it will be assumed that the light polarizationconfiguration for observing the display in transmission, in reflectionor in a hybrid mode is optimized to the design for giving the bestpossible off state in the pixels. The off state may correspond either tothe twisted texture or to the uniform texture. To take an example,optimization of the polarization configuration in reflection mode ispreferably in accordance with the provisions defined in the patentapplication filed by the Applicant on 12 May 2000 under No. 00/06107.

According to the present invention, the bistable nematic liquid-crystaldisplay device comprises, for forming an inter-pixel optical mask:

-   -   a) a nematic liquid-crystal material contained between two        parallel substrates, which are provided with electrodes on their        facing inner surfaces in order to allow an electric field to be        applied to the said liquid crystal, at least the front substrate        and the front electrode being optically transparent;    -   b) alignment layers or treatments on the electrodes which orient        the liquid crystal and produce an alternation of at least two        distinct textures that are stable or metastable in the absence        of a field, where one of the textures is either not twisted or        twisted through a total angle of between −90° and +90° and the        other texture has an additional twist through an angle of about        180°;    -   c) the thickness d of the liquid-crystal layer covering the        electrodes being chosen so that the product d.Δn is close either        to λ_(0/4) or λ_(0/2), to within an integer multiple of the        wavelength, or else is between these two values (depending on        whether the operating mode of the display is reflective,        transmittive or transflective), λ₀ being the median wavelength        of the useful spectral band of the display and Δ_(n) the        birefringence of the liquid crystal for this wavelength;    -   d) means designed to apply, to the liquid crystal covering the        electrodes, electrical signals which allow it to switch, by        breaking the anchoring on at least one of the two substrates,        between the said distinct textures and allow it to remain in one        of them after the field has been removed; and    -   e) means designed to promote the establishment of that one of        the said textures giving the off state in the liquid crystal        covering the inter-electrode spaces.

It should be understood that the number and the arrangement of thepolarizers are optimized according to the optical operating mode(transmissive, reflective or transflective) of the display device.

According to other advantageous features of the present invention:

-   -   at least one of the electrodes contains several different        segments in order to make it possible to produce several        independent picture elements (pixels) on the same substrate and        in the same device;    -   the independent picture elements (pixels) are provided with        independent means for applying the field;    -   the independent picture elements (pixels) are organized in a        multiplexed passive matrix; or    -   the independent picture elements (pixels) are organized in a        multiplexed active matrix.

Further features, objects and advantages of the present invention willbecome apparent on reading the detailed description which follows and inconjunction with the appended drawings, given as non-limiting examplesand in which:

FIG. 1, described above, shows schematically a conventional pixelobtained at the intersection of two mutually orthogonal electrodes;

FIG. 2, described above, shows schematically a multiplexed matrix formedby an assembly of two crossed arrays of electrodes, the electrodes beinglocated on each side of a liquid-crystal material, defining pixels andinter-electrode spaces;

FIG. 3, also described above, shows schematically a sectional view ofsuch a multiplexed matrix;

FIG. 4 illustrates schematically the stability of the textures as afunction of the spontaneous twist in the thickness of the liquidcrystal;

FIG. 5 shows schematically the induction of a twisted texture in aninter-pixel gap in accordance with the present invention, by the removalof material, leading to stable parallel (P_(A)) or twisted (T_(W))textures (and therefore to an instability) in the pixels and to apreferential twisted texture in the inter-pixel gap; and

FIG. 6 shows schematically the induction of a parallel texture in aninter-pixel gap in accordance with the present invention, by addingmaterial, leading to stable parallel (P_(A)) or twisted (T_(W)) textures(and therefore to an instability) in the pixels and to a preferentialparallel texture in the inter-pixel gap.

The displays forming the subject of the present invention can addresstwo stable textures on the liquid-crystal layer covering their pixels.Bistability is achieved by adding a chiral dopant to the liquid crystal.

Without a chiral dopant, a liquid crystal has a uniform orientation inthe absence of torque. Its free pitch is infinite. In the presence of achiral dopant, the orientation of the liquid crystal rotates along itshelix when it is traversed along its axis. The liquid crystal then has afinite free pitch (the orientation direction rotates through 360° aftera displacement along the axis of a distance equal to the pitch). Thefree pitch decreases as the dopant concentration increases. The freetwist q₀ of the liquid crystal is defined by the expression 2π/p₀ wherep₀ is the free pitch of the liquid crystal, and the spontaneous twistafter the traversing of a specimen of the liquid crystal having athickness d is defined by d.q₀. The spontaneous twist is therefore theangle through which the director would have rotated after traversing thelayer of liquid crystal in the absence of any torque.

Bistability is obtained by adjusting the spontaneous twist in thethickness of the liquid crystal covering the pixels to a value of 90°.The conditions of anchoring on the surfaces, in the two possible—twistedand uniform—textures dictate that the liquid crystal is deformed by atwist of about 90° with respect to its equilibrium state. The energiesof the two states are then equal and the two textures become bistable. Aspontaneous twist of less than 90° favours the uniform texture while aspontaneous twist of more than 90° favours, on the other hand, thetwisted texture. We should note at this stage that the fine features ofthe anchoring on the surfaces also contribute to the energy of thetextures. For example for the same 90° spontaneous twist of the liquidcrystal in the thickness of the cell, an anchoring configuration with asufficiently high parallel pretilt will favour the twisted texture,whereas a configuration with the same but antiparallel pretilt willfavour the uniform texture. In practice, strict balancing of theenergies is not absolutely essential for ensuring bistability, becauseof the existence of a potential barrier preventing the spontaneoustransition from one texture to the other. For example, the spontaneoustwist may be adjusted over a certain range centred on 90° without lossof bistability (the twisted and uniform textures both remaining stable).However, outside this range, the texture of minimum energy is the onlyone which is stable (see FIG. 4).

The means developed by the present invention for establishing thetexture giving the off state in the liquid crystal covering theinter-pixel gap will now be presented.

The first means is to make the thicknesses of the liquid crystal in theactive portions (pixels) and the non-active portions (inter-pixel gaps)different, this thickness difference being adjusted by selective removalor addition of material. Since the spontaneous twist in the pixels mustbe close to 90°, in order to make the twisted and uniform texturestherein of equal energy, the localized addition or removal of materialin the inter-electrode spaces of each substrate decreases or increasesthe spontaneous twist in the thickness of the liquid crystal in theinter-pixel gaps. The difference in twist is proportional to thedifference in the thicknesses of the liquid crystal covering the pixelsand the inter-pixel gaps. When the twist in one place differssignificantly from the 90° ideal equilibrium twist, the bistability islost and one of the two textures is favoured (a difference of around 5to 1020 is sufficient for example to favour one texture). According tothis first means, all that therefore is required is to set bymanufacture the said thickness difference to a value sufficient to makethe texture giving the on state unstable (it being possible for thistexture to be either the uniform texture or the twisted texture,depending on the polarization configuration adopted).

If it is the twisted texture which gives the off state, a slightrecessing of the inter-electrode space on at least one substrate favoursit and establishes it in the inter-pixel gap, thus forming aliquid-crystal optical mask (see FIG. 5).

If it is the uniform texture which gives the off state, localizeddeposition of material in the inter-electrode space, on at least onesubstrate, allows the same result to be achieved (see FIG. 6).

For example, let us take the case of a display in reflection, providedwith a circular polarizer on the front face and a reflector on the rearface, the display being filled with a liquid crystal having abirefrigence Δn≈0.13. The thickness d of liquid crystal covering thepixels is chosen to be 1.1 μm, similar to the optimum thickness for thereflective case given by the equation d·Δn=λ_(0/4) where λ₀ is themedian wavelength of the spectral operating region. The twisted texturetherefore gives the off state. The spontaneous equilibrium twist givingbistability to the pixels is adjusted by manufacture to 90°. Byrecessing the inter-electrode spaces by 0.1 μm on each substrate, thespontaneous twist becomes approximately 98° in the thickness of theliquid crystal covering the inter-pixel gaps (equal to 1.2 μm—here weneglect the case of the small regions where the inter-electrode spacesof the two substrates intersect). This twist is sufficient to favour thetwisted texture being established. An off state optical mask having alow degree of reflection surrounding the pixels is therefore formed.

The thickness of the liquid crystal in the inter-pixel gap must,however, remain sufficiently close to that in the pixels so that theoptical state of the inter-pixel gap remains the off state. Thiscondition is satisfied when the difference in thickness of the liquidcrystal covering the pixels and the inter-pixel gaps does not exceedapproximately 15% of the thickness of the liquid crystal covering thepixels. This condition determines the maximum amplitude of the internalrelief of the display, more specifically that of the alignment surfacedeposited on the electrodes or inter-electrode spaces, it being possiblefor this relief to be in the form of recesses or projections. The choiceof recessed or added material in the inter-electrode space and thechoice of etching or deposition method are guided by considerations ofcompatibility with the usual manufacturing process for displays. Forexample, if a plasma etching means is used to manufacture theelectrodes, the slight recessing of the inter-electrode space requiresneither an additional mask nor a new operation (however, it may provenecessary to adapt the etching parameters).

Another means developed by the present invention for forming theliquid-crystal optical mask applies to the case in which the off stateis given by the twisted texture. This means consists in manufacturingelectrodes of thickness adapted to the creation of the relief allowingthe twisted texture to be established in the inter-pixel gap. However,this method requires relatively thick electrodes (for example with athickness of about 40 nm or more) to be manufactured. This is sometimesincompatible with compliance with the optical or electricalspecifications of the display or with certain manufacturing constraints.

Another method for inducing the liquid-crystal optical mask is to adjustthe chiral dopant content of the liquid crystal until the appearance ofthe desired texture is obtained in the inter-pixel gap, the displaybeing designed so that the thickness of the liquid crystal covering thepixels and the inter-pixel gaps is slightly different. The potentialbarrier which exists between the twisted and uniform textures gives infact a certain spread to the range of spontaneous twists compatible withbistability in the pixels. For example, the chiral dopant content may beincreased up to the point of reaching the limit of bistability in thepixels without exceeding it. Any recessed relief, even barelypronounced, in the inter-pixel gap compared with the pixel makes thespontaneous twist exceed this limit, it thereby being sufficient toselectively establish therein the twisted texture. The same methodapplies to establishing the uniform texture in an inter-pixel gap havinga projecting relief, by reducing the chiral dopant content down to thelimit value.

Another method is to create anchoring conditions specific to theliquid-crystal layer covering the inter-pixel spaces which favour thetexture giving the off state. For example, the alignment layer may bedeposited in the liquid phase in such a way that it has an additionalthickness in the recessed inter-electrode spaces compared with itsthickness on the surface of the electrodes, thereby reducing itsazimuthal anchoring energy. This effect, in conjunction with the locallowering of the brushing force applied in these recessed surfaces,favours the twisted texture being established by the orientationdirection slipping under the influence of the azimuthal torque exertedby the liquid-crystal layer. This result may be accentuated by a heattreatment of the display. It is also possible, by carrying out anorientation treatment in two goes, to selectively modify the orientationdirection in the inter-electrode space so that the spontaneous twist inthe liquid-crystal layer covering these spaces is unfavourable to thetwisted or uniform texture therein. In the same way, selectivemodification of the pretilt may allow the same result to be achieved.

Finally, the use of temporary treatment may allow the texture giving theoff state to be established in the inter-pixel gap, even when theinternal relief is slight, by momentarily favouring the desired texture.For example, if the display is filled with a provisional thickness ofliquid crystal slightly greater than the final thickness, thespontaneous twist temporarily favours the twisted state beingestablished both in the pixels and the inter-pixel gaps. When the layerof liquid crystal is reduced to its nominal thickness at the end of themanufacturing operations, bistability in the pixels is obtained. If therecessed relief in the inter-pixel gaps is sufficient, it has beenobserved that the uniform texture is unstable and the twisted texture isfavoured. If the relief is too slight (for example when it is caused bythe etching of thick electrodes by less than 40 nm), the inter-pixelgaps are also bistable and preserve the twisted state initiallyestablished by the temporary treatment. This state is no longer affectedby the subsequent operation of display and a definitive optical mask isobtained.

Another means is to carry out a heat treatment of the display afterfilling, often allowing the same result to be obtained.

When one of the methods explained above is employed, the liquid crystalitself blocks off the non-addressable regions of the display and forms aliquid-crystal optical mask easily achieving a contrast value of greaterthan several tens.

The favourable properties of this self-aligned liquid-crystal opticalmask are obtained whether the operation is transmissive, reflective ortransflective and they have many advantages compared with the otherblocking techniques already discussed in which opaque layers aredeposited and etched:

-   -   low intrinsic degrees of transmission and reflection of the        liquid-crystal optical mask, these being adapted to the        production of high-contrast transmissive, reflective or        transflective displays;    -   no degradation in the off state by partial blocking of the        pixels by an added optical mask (maximum open aperture ratio);    -   no degradation in the homogeneity of the electric field within        the display;    -   no disturbance of the alignment of the liquid crystal outside        the image refreshing periods and therefore no degradation in the        contrast when the stored image is observed; and    -   no operation of depositing or of aligning the liquid-crystal        optical mask.

Although the above methods have been presented taking the example ofdisplays in the form of passive matrices, they can be applied to anyparticular arrangement of electrodes of the two substrates resulting inthe definition of addressable regions (pixels) or non-addressableregions (inter-pixel gaps).

BIBLIOGRAPHY

[1] A. Lien, “Two dimensional Simulation of the Lateral Field Effect ofa 90 degree TN-LCD cell”, IDRC Eurodisplay '90 Digest.

[2] European Patent EP 0 532 311 (Scheffer) published on 17 Mar. 1993.

[3] U.S. Pat. No. 5,307,189 (Nishiki) published on 26 Apr. 1994.

[4] U.S. Pat. No. 5,666,177 (Hsieh Ting-Chang) published on Sep. 9,1997.

[5] U.S. Pat. No. 5,686,980 (Kobayashi) published on Nov. 11, 1997.

[6] French Patent No. 2 740 894 (Durand).

1. Bistable nematic liquid-crystal display device with a liquid-crystaloptical mask, characterized in that it comprises: a) a nematicliquid-crystal material contained between two parallel substrates, whichare provided with electrodes on their facing inner surfaces in order toallow an electric field to be applied to the said liquid crystal, atleast the front substrate and the front electrode being opticallytransparent; b) alignment layers or treatments on the electrodes whichorient the liquid crystal and produce an alternation of at least twodistinct textures that are stable or metastable in the absence of afield, where one of the textures is either not twisted or twistedthrough a total angle of between −90° and +90° and the other texture hasan additional twist through an angle of about 180°; c) the thickness dof the liquid-crystal layer covering the electrodes being chosen so thatthe product d.Δn is close either to λ/4 or λ/2, to within an integermultiple of the wavelength, or else is between these two values, λ₀being the median wavelength of the useful spectral band of the displayand Δn, the birefringence of the liquid crystal for this wavelength; d)means designed to apply, to the liquid crystal covering the electrodes,electrical signals which allow it to switch, by breaking the anchoringon at least one of the two substrates, between the said distincttextures and allow it to remain in one of them after the field has beenremoved; and e) means designed to promote the establishment of that oneof the said textures giving the off state in the liquid crystal coveringthe inter-electrode spaces.
 2. Device according to claim 1,characterized in that it furthermore includes a chiral dopant whosecontent is adjusted in such a way that the spontaneous twist in thethickness of the liquid crystal covering the inter-electrode spaces isunfavourable to the texture giving the on state and in that thespontaneous twist in the thickness of the liquid crystal covering thepixels lies within the range allowing both these textures to bebistable.
 3. Device according to either of claims 1 and 2, characterizedin that it has a difference in thickness of liquid crystal between theactive portions corresponding to the pixels and the non-active portionscorresponding to the inter-pixel gaps.
 4. Device according to claim 3,characterized in that the difference in thickness of the liquid crystalis produced by localized addition or removal of material in theinter-electrode spaces of each substrate in such a way that thespontaneous twist in the thickness of the liquid crystal covering theinter-electrode spaces is unfavourable to the texture giving the onstate.
 5. Device according to claim 1, characterized in that it includeselectrodes with a thickness such that the spontaneous twist in thethickness of the liquid crystal covering the inter-electrode spaces isunfavourable to the texture giving the on state.
 6. Device according toclaim 1, characterized in that the means for favouring establishment ofthe texture giving the off state comprise an alignment or anchoringcharacteristics, in the inter-electrode spaces, adjusted so as to favourthe texture giving the off state therein.
 7. Device according to claim1, characterized in that the means to favour the texture giving the offstate comprise a temporary treatment of the electrical, mechanical orheat type, associated with anchoring characteristics such that thetemporary treatment is unfavourable to the texture giving the on statein the inter-electrode spaces.
 8. Device according to claim 1,characterized in that at least one of the electrodes contains severaldifferent segments in order to make it possible to produce severalindependent picture elements (pixels) on the same substrate and in thesame device.
 9. Device according to claim 1, characterized in thatindependent picture elements (pixels) are provided with independentmeans for applying the field.
 10. Device according to claim 1,characterized in that independent picture elements (pixels) areorganized in a multiplexed passive matrix.
 11. Device according to claim1, characterized in that independent picture elements (pixels) areorganized in a multiplexed active matrix.
 12. Process for manufacturinga bistable nematic liquid-crystal display device with a liquid-crystaloptical mask, characterized in that it comprises the steps consistingin: a) placing a nematic liquid-crystal material between two parallelsubstrates, which are provided with electrodes on their facing innersurfaces in order to allow an electric field to be applied to the saidliquid crystal, at least the front substrate and the front electrodebeing optically transparent; b) forming alignment layers or treatmentson the electrodes which orient the liquid crystal and produce analternation of at least two distinct textures that are stable ormetastable in the absence of a field, where one of the textures iseither not twisted or twisted through a total angle of between −90° and+90° and the other texture has an additional twist through an angle ofabout 180°; c) controlling the thickness d of the liquid-crystal layercovering the electrodes so that the product d.Δn is close either to λ/4or λ/2, or else is between these two values, λ₀ being the medianwavelength of the useful spectral band of the display and Δn thebirefringence of the liquid crystal for this wavelength; d) providingmeans designed to apply, to the liquid crystal covering the electrodes,electrical signals which allow it to switch, by breaking the anchoringon at least one of the two substrates, between the said distincttextures and allow it to remain in one of them after the field has beenremoved; and e) providing means designed to promote the establishment ofthat one of the said textures giving the off state in the liquid crystalcovering the inter-electrode spaces.
 13. Process according to claim 12,characterized in that it comprises the step consisting in producing adifferent thickness of liquid crystal between the active portionscorresponding to the pixels and the inter-pixel non-active portions. 14.Process according to claim 13, characterized in that the difference inthickness between the active and non-active portions is less than 15% ofthe thickness of the liquid crystal covering the active portions. 15.Process according to either of claims 13 and 14, characterized in thatthe difference in thickness of the liquid crystal is between 5 and 10%of the thickness of the liquid crystal covering the active portions. 16.Process according to claim 13, characterized in that the difference inthickness of the liquid crystal is produced by removing material betweenthe active portions.
 17. Process according to claim 16, characterized inthat the removal of material is controlled in order to define a twistedtexture corresponding to the off state.
 18. Process according to claim16, characterized in that the localized removal of material in theinter-electrode spaces of each substrate is carried out during the sameoperation as the etching of the electrodes, by a wet etching orplasma-enhanced etching process.
 19. Process according to claim 13,characterized in that the difference in thickness of the liquid crystalis obtained by adding material in the non-active portions.
 20. Processaccording to claim 19, characterized in that the addition of material iscontrolled so as to obtain a uniform texture corresponding to the offstate in the inter-electrode spaces.
 21. Process according to claim 13,characterized in that the difference in thickness of liquid crystal isobtained by controlling the thickness of the electrodes in such a waythat the spontaneous twist in the thickness of the liquid crystalcovering the inter-electrode spaces is unfavourable to the texturegiving the on state.
 22. Process according to claim 12, characterized inthat it includes a step consisting in adjusting the chiral dopantcontent of the liquid crystal in such a way that the spontaneous twistin the thickness of the liquid crystal covering the inter-electrodespaces is unfavourable to the texture giving the on state and in thatthe spontaneous twist in the thickness of the liquid crystal coveringthe pixels lies within the range allowing the twisted and uniformtextures to be bistable.
 23. Process according to claim 22,characterized in that the device has a recessed inter-pixel relief andin that the step of adjusting the chiral dopant content consists inincreasing this content until a bistability limit is obtained. 24.Process according to claim 22, characterized in that the device includesprojecting inter-pixel reliefs and in that the step of adjusting thechiral dopant content consists in reducing this content until abistability limit is obtained.
 25. Process according to claim 24,characterized in that it includes a step consisting in depositing analignment layer in the liquid phase in such a way that it has anadditional thickness in the recessed inter-electrode spaces comparedwith its thickness on the surface of the electrodes, so as to reduce itsazimuthal anchoring energy.
 26. Process according to claim 12,characterized in that it includes the step consisting in controlling theanchoring or alignment characteristics in the inter-electrode spaces soas to favour the textures giving the off state therein.
 27. Processaccording to claim 26, characterized in that it includes an orientationtreatment carried out in two goes so as to selectively modify theorientation direction in the inter-electrode space so that thespontaneous twist in the liquid crystal layer covering these spaces isunfavourable to the on state texture therein.
 28. Process according toclaim 12, characterized in that it includes an electrical, mechanical orheat treatment of the device so as to induce a temporary modification ofthe spontaneous twist and to control the anchoring characteristics inthe liquid crystal covering the inter-electrode spaces, such that thistemporary treatment is unfavourable to the texture giving the on statetherein.
 29. Process according to claim 12, characterized in that thefilling operation is carried out with a thickness of liquid crystalunfavourable to the texture giving the on state, theend-of-manufacturing operations then giving the layer of liquid crystalfilling the device its nominal thickness.
 30. Process according to claim29, characterized in that the operation of filling the device is carriedout with a thickness of liquid crystal greater than the final nominalthickness, the layer of liquid crystal then being reduced to its nominalthickness, at the end of the manufacturing operations, in order toobtain bistability in the pixels while maintaining the textureunfavourable to the on state between the pixels.
 31. Process accordingto claim 12, characterized in that at least one of the electrodescontains several different segments in order to make it possible toproduce several independent picture elements (pixels) on the samesubstrates and in the same device.
 32. Process according to claim 12,characterized in that the independent picture elements (pixels) areprovided with independent means for applying the field.
 33. Processaccording to claim 12, characterized in that the independent pictureelements (pixels) are organized in a multiplexed passive matrix. 34.Process according to claim 12, characterized in that the independentpicture elements (pixels) are organized in a multiplexed active matrix.